If it doesnt get completely better soon, i would go and have the vet check it out. Little dogs are very good at coping on 3 legs and just putting a bit of pressure on the injured one. Molly did this and after a mis-diagnosis by a junior vet,2 weeks later i asked to see the senior one and he straight way said she had a partially torn cruciate ligament!
fortunately i had ignored the junior vets advice to let Molly do as much as she wanted, and had been making her take it easy.
8 weeks later and her knee is on the mend. we are taking short walks, but no off-lead exercise for another 2 months. It will be 6 months till she can run around as she pleases, and she will almost certainly now get arthritis in that knee.
dont let it go on for too long before you get it checked....good luck and i hope it turns out to be nothing serious..xx